Use of an electricity storage device such as a secondary battery enables effective use of energy. For example, in recent years, the development of solar power generation systems for environmentally friendly clean energy has been energetically pursued. However, photoelectric conversion modules for converting solar light into electric power have no electricity storing functions. In some case, therefore, a photoelectric conversion module is used in combination with a secondary battery. For example, effective use of energy is made by charge/discharge control performed by temporarily storing electric power generated by a photoelectric conversion module in a secondary battery and by discharging the power from the secondary battery according to an external load requirement or the like.
For example, patent document 1, a document describing an art relating to the present invention, discloses a solar cell power supply apparatus having solar cells, a plurality of secondary batteries to be charged with electricity from the solar cells, charge switches connected between the secondary batteries and the solar cells to control charging of the secondary batteries, discharge switches connected between the secondary batteries and a load, and a control circuit that controls the charge and discharge switches. This document discloses a process in which a control circuit determines priorities with which charging of the secondary batteries is performed by controlling the plurality of charge switches, charges one of the secondary batteries with a higher priority earlier than it charges another of the secondary batteries with a lower priority, and charges the other of the secondary batteries with the lower priority after the completion of charging of a certain capacity in the secondary battery with the higher priority.